LOW BATTERY

after i have been playing a game for about 2 hours i run out of memory,have to re-boot then everything ok, my clock also looses time.It has been suggested to me that it could be my battery, i presume thats the battery in my motherboard.Also been told its a big job to replace it,dont know much about it, just thought it might be easier to get a new motherboard .Please help a damsel in distress